Characteristics of Cu Adsorption in Aggregate size fractions and its relations with some soil properties

Background and Objectives: The heavy metals accumulate in the soil through adsorption, precipitation and other physical and chemical processes. The absorbed metals in the soils can enter into rivers or groundwater by runoff and can concentrate in animals, plants, and humans. The soil aggregates are units of soil structure. The soil aggregates with different size can differently adsorb and transfer the heavy metals such as copper (Cu). Accordingly, information about the fate of Cu in the soil aggregates is important for understanding the effects of Cu in soils. Heterogeneous Cu adsorption in various aggregate fractions influences availability and mobility of this metal. Therefore, ability of aggregate size fractions in sorption of Cu is important for agricultural management, watershed modeling, and environmental research. This paper aims at investigating Cu adsorption in 5 calcareous soils. This goal is of particular importance since although it is important to study Cu adsorption in aggregate size fractions, little research has been proceed to Cu adsorption studies in aggregates. The objectives of this study were (i) to evaluate the Cu adsorption in different aggregate size fractions (ii) to evaluate different models for describing Cu adsorption and (iii) to estimate relation between Cu adsorption characteristics in different aggregate size fractions and soil properties.
Materials and Methods: For separation of soil aggregates of 5 selected calcareous soils from Chaharmahal –va- Bakhtiari Province used the dry sieving method. The soils were divided into 4 sections including aggregates larger than 2 mm, 2 to 0.25 mm, 0.25 to 0.053 mm and smaller than 0.053 mm, and larger aggregates than 0.25 mm and smaller aggregates than 0.25 mm were named macro and microaggregates, respectively. Also, properties including free Fe oxides, OC, CEC, CCE, EC, pH were determined in each aggregate. Then, the Langmuir and Freundlich, and linear were fitted on the adsorption data. To determine relation between properties of Cu adsorption and properties of aggregates used correlation coefficient (r) and regression.
Results: Based on the results of the properties of soil aggregates free iron oxides in the macroaggregates were higher than microaggregates. While OC, CEC, and CCE in the macroaggregates were lower than microaggregates. The results of Cu adsorption showed that maximum of Cu adsorption (b in Langmuir equation), maximum buffering capacity (MBC in Langmuir equation) and distribution coefficient (kf and B in Linear and Freundlich equations) in microaggregates were higher than macroaggregates (P<0.05). While, energy of adsorption (k in Langmuir and n in Freundlich equations) in microaggregates were lower (P<0.05) than microaggregates. The results of correlation study revealed that pH, CEC, and CCE were the most effective soil properties on parameters of adsorption equations.
Conclusion: The results of this study showed that the microaggregates (smaller than 0.25 mm) with higher capacity of Cu adsorption and lower power compared to macroaggregates (larger than 0.25 mm) have the potential to accumulation of Cu, therefore transfer this part of soils by different processes can entire Cu pollution to other places.